Priority is claimed to Patent Application Numbers 2000-86369, filed in the Republic of Korea on Dec. 29, 2000, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a phase-conjugate holographic data storage device using a multifocal lens, and more particularly, to a phase-conjugate holographic data storage device capable of enhancing data recording and reproducing characteristics and data recording density by using a multifocal lens.
2. Description of the Related Art
Holography is a technique for reproducing the original form of optical signals, that is, a technique for recording a certain pattern resulting from an interference phenomenon between an object beam reflected by an object and a reference beam traveling straight at a different angle by taking advantage of the characteristics of a laser beam and then reproducing a three dimensional image of the object using the recorded pattern.
In holography, a laser beam having a uniform mono-waveform is divided into two beams using a beam splitter such as a semitransparent mirror. One of the two beams is flashed on an object so as to generate an object beam and then, the object beam is caused to interfere with the other beam, thereby creating an interference pattern. After that, the interference pattern is recorded in a storage medium. In reproducing a three dimensional image of the object using the interference pattern, a reference beam is transmitted through the storage medium.
Specifically, a holographic data storage device is used for recording data by transforming optical signals differently modulated depending on types of data into an object beam and reproducing the data. The object beam interferes with a reference beam so as to make an interference pattern. The interference pattern is recorded in a solid formed of material which reacts according to the intensity of the interference pattern (a storage medium) and the interference pattern recorded in the storage medium is called a hologram. If the reference beam is applied to such a hologram, the original form of the object beam used in the recording is reconstructed. Here, the hologram data recorded in the storage medium can be read by using only the reference beam used in recording because a reference beam having a different wavelength or phase is not diffracted by the hologram recorded in the storage medium but just passes through the hologram. Such a holographic technique has been applied to advertising, marketing, industry, exhibitions, packaging or decoration, and new technical applications are being developed.
A typical phase-conjugate holographic data storage device will be described with reference to FIGS. 1A and 1B. FIG. 1A is a diagram illustrating the data recording principle of a typical phase-conjugate holographic data storage device and FIG. 1B is a diagram illustrating the data reproducing principle of a typical phase-conjugate holographic data storage device.
Referring to FIG. 1A, a focusing lens 11 is placed in the path of an object beam 15. A data recording medium 13 is placed at a focal spot into which the object beam 15 converges. A spatial light modulator (SLM) 12 is placed between the focusing lens 11 and the data recording medium 13 on the incidence path of the object beam 15 and is closer to the focusing lens 11. The SLM 12 modulates an optical signal spatially depending on the type of data, and the focusing lens 11 reduces the space band width of the modulated optical signal.
The data storage principle of the phase-conjugate holographic data storage device is as follows. As shown in FIG. 1A, in recording data, the object beam 15 passes through the focusing lens 11 and then is modulated by the SLM 12. The modulated object beam 15 is connected to the storage recording medium 13 and focused by the focusing lens 11 and thus, its space band width is reduced. At this time, a reference beam 16a interferes with the object beam 15 and the interference pattern is recorded in the data recording medium 13. As shown in FIG. 1B, in reading data, a reference beam 16b is applied to the data recording medium 13. As a result, the interference pattern recorded in the data recording medium 13 is reconstructed, output and then sensed by an image sensor 14. In such a mechanism of recording and reproducing data, if a monofocal lens is used in focusing an object beam including data, as is the case in the prior art, light intensity, that is, a direct current (DC) term increases significantly in the middle of the Fourier plane at which the object beam is focused. Thus, the characteristics of recording and reproducing data are deteriorated.
Methods of solving this problem which have been used in the prior art, will be described with reference to FIGS. 2 and 3. Referring to FIG. 2, a data recording medium 23 is arranged a predetermined distance apart from the focal plane of a focusing lens 21. An object beam 24 is spatially modulated by a SLM 22 and is focused by the focusing lens 21. After that, the object beam 24 and a reference beam 25a interfere with each other, thereby creating an interference pattern. The interference pattern is recorded in the data recording medium 23. In this case shown in FIG. 2, the data recording medium 23 is arranged at a distance of several millimeters to several centimeters from the focal spot of the object beam 24 induced by the focusing lens 21. As a result, the focal plane on which the object beam 24 passing through the focusing lens 21 and the SLM 22 is focused is not included in an area in which the interference phenomenon occurs and thus, the size of a signal beam in recording data increases, so that the light intensity in the middle of the Fourier plane, that is, the DC term cannot affect the characteristics of recording and reproducing data.
In the case of reproducing data recorded in the data recording medium 23, the reference beam 25a is applied to the data recording medium 23. As a result, a phase-conjugate signal beam is reproduced and then is detected by an image sensor (not shown).
FIG. 3 is a diagram of a conventional phase-conjugate holographic data storage device in which a phase mask 34 is additionally installed between a SLM 32 and a data recording medium 33. Referring to FIG. 3, the phase mask 34 is placed on an optical path along which an object beam 35 is spatially modulated by the SLM 32 and is focused at a focal plane. As a result, in the case of recording data, the object beam 35 sequentially passing through the focusing lens 31, the SLM 32 and the phase mask 34 interferes with a reference beam 36a, thereby generating an interference pattern. The interference pattern is recorded in the data recording medium 33. If the object beam passes through the phase mask 34, the DC term occurring in the middle of the Fourier plane is weakened. In the case of reproducing data, the phase mask 34 is not necessary.
However, in the case of the holographic data storage device of FIG. 2, focusing of the object beam 24 and the interference phenomenon induced by the object beam 24 and the reference beam 25b occur at different locations. Accordingly, the holographic data storage device has a problem in that the size of a spot to be recorded becomes greater than the size of the spot on the Fourier plane and thus, data recording density becomes smaller than that of the holographic data storage devices of FIGS. 1A and 1B. In the case of the phase-conjugate holographic data storage device shown in FIG. 3, it is required to install the phase mask 34 specially designed between the SLM 32 and the data recording medium 33 and thus, the manufacturing cost of the data storage device increases. In addition, the phase-conjugate holographic data storage device has another problem in that the phase mask 34 should be aligned very precisely.
To solve the above problems, it is an object of the present invention to provide a phase-conjugate holographic data storage device which is capable of maintaining data storage density by decreasing a direct current term, which is light intensity at the center of the Fourier plane, and is economical and a data storage method thereof.
Accordingly, to achieve the above object, there is provided a holographic data storage device comprising: a multifocal lens which focuses an object beam having data; a spatial light modulator which is located on the optical path of the object beam passing through the multifocal lens and modulates the object beam; and a data recording medium which records an interference pattern generated by interference between a reference beam and the object beam passing through the spatial light modulator and converging into a focus. Here, the multifocal lens is a solution focusing lens having at least two different focuses.
Preferably, the data storage medium is formed of a photorefractive crystal or optical polymer which is capable of storing light in terms of a refractive index.
According to another aspect of the present invention, there is provided a method for recording holographic data in a holographic data storage device comprising a focusing lens focusing an object beam with data and having at least two different focuses, a spatial light modulator located on the optical path of the object beam passing through the focusing lens and modulating the object beam, and a data recording medium recording an interference pattern generated by interference between a reference beam and the object beam passing through the spatial light modulator and converging into a focus, the method comprising: modulating the object beam by using the spatial light modulator; converging the spatially modulated object beam at at least two focuses by using the focusing lens; and recording an interference pattern generated by interference between a reference beam and the object beam passing through the spatial light modulator and converging into a focus in the data recording medium.
Preferably, the data storage medium is formed of a photorefractive crystal or optical polymer which is capable of storing light in terms of a refractive index.